In 2005, 3GPP (The 3rd Generation Partnership Project) launched the Long Term Evolution (LTE) research project which is intended to provide support for increasingly growing demands from operators and users by a higher data throughput and a better network performance.
Multimedia Broadcast/Multicast Service (MBMS) is a service introduced by 3GPP Rel 6, which refers to a point-to-multipoint service in which one data source transmits data to multiple users, so that network (including core network and access network) resource sharing can be achieved. In this way, services for as many as possible users having the same demand can be satisfied by using resources as few as possible. In a wireless access network, by utilizing common transmission channels and common radio bearers, MBMS enables multicast and broadcast of not only low rate plain-text message, but also high rate multimedia service, such as mobile TV.
Various researches on Evolved MBMS (EMBMS) are currently ongoing.
In 3GPP LTE, the core network employs a two-layer flat network architecture in which four major network elements at the original WCDMA/HSDPA stage, i.e., NodeB, RNC, SGSN and GGSN, are evolved into two major network elements, i.e., eNodeB (eNB, evolved NodeB, referred to as “base station” hereinafter) and access gateway (GW). Further, the core network utilizes an all-IP distributed structure to support IMS, VoIP, SIP, Mobile IP, etc.
FIG. 1 illustrates the structure of an LTE network. An access gateway, GW, receives data from a Broadcast Multicast Service Center (BM-SC). The access gateway GW is connected with a plurality of base stations eNB1, eNB2 and eNB3. The user plane interface between the access gateway GW and one of the base stations is referred to as M1 interface. The plurality of base stations (eNBs) is connected to one another in a mesh form (indicated by dashed line in FIG. 1). The interface between two of the base stations is referred to as X2 interface. A plurality of user equipments (UEs), UE 11˜UE 12, UE 21˜UE 23 and UE 31˜UE 33, are illustratively shown in the cells serviced by the base stations eNB1-eNB3, respectively.
In LTE, the downlink transmission scheme at the physical layer of the wireless interface is OFDM scheme, and the uplink transmission scheme is SC-FDMA scheme. Due to the OFDM mechanism, the same wireless signals from different cells can be spontaneously combined in the air to improve signal strength, such that Radio Frequency (RF) combining can be performed without additional processing overhead.
Thus, as for EMBMS in LTE, it has been defined as a baseline requirement to support RF combining in the air in the Single Frequency Network (SFN) multi-cell transmission mode, so as to improve the gain at the border of the cell, since EMBMS needs to transmit the same service data to several different users.
In SFN, base stations (eNBs) have been synchronized in terms of frame timing at the physical layer, and the accuracy of the synchronization can satisfy the requirement for EMBMS RF combining. However, in order to ensure the effectiveness of RF combining, the wireless signals to be combined have to be kept synchronous with and consistent with one another in terms of MBMS service content. That is, the transmission synchronization at Layer 2 (L2) needs to be guaranteed for multi-cell transmission of MBMS service.
Further, in the LTE network architecture, IP multicast transmission has been design to extend to the base station eNB. With IP multicast, an MBMS service packet can be transmitted to a group of base stations (eNBs) at a time. The current routing protocol for IP multicast specifies that the route from each base station eNB to an access gateway GW mainly depends on the current network topology and remains unchanged unless a related router fails (which rarely happens). In addition, the processing capabilities of the routers in the network as well as the transmission load of the network will be optimized during network optimization. Thus, the major factor affecting different transmission delays over different transmission paths from different base stations eNBs to the access gateway GW generally consists only in the difference of the transmission paths. In other words, while the base stations (eNBs) in an SFN area have been synchronized in terms of frame timing at the physical layer, the same MBMS data packet may arrive at different base stations (eNBs) at different timings due to the different routes.
FIG. 2 is a schematic diagram illustrating delays for the same data packet transmitted from the same access gateway to different base stations. As shown in FIG. 2, the data packet is transmitted from the access gateway GW to the base station eNB 1 via a route of access gateway GWrouter R1base station eNB 1. On the other hand, the data packet is transmitted from the access gateway GW to the base station eNB2 via a route of: access gateway GWrouter R2router R3base station eNB2. The different routes via which the same data packet are transmitted will necessarily cause different delays.
As shown in FIG. 2, the data packet is transmitted from the access gateway GW to the base stations eNB1 and eNB2 at T0 simultaneously, but arrives at the base station eNB1 at T1 and arrives at the base station eNB2 at T2, respectively. In this case, the same data packet arrives at different base stations (eNBs) with a relative delay of TD T2−T1.
It can be seen from the above, if the base stations eNB1 and eNB2 hardly receive the data packets from the access gateway GW when they transmit these data packets out, it is obvious that the data packets having the same content will be transmitted to user equipments UEs from different base stations (eNBs) asynchronously. As a necessary result, these data packets cannot be correctly combined or even introduce additional interference. Moreover, after the arrival of the same data packet, each base station eNB needs to perform processes (such as segmentation, encoding, modulation and framing, etc.) on the packet. In this case, inconsistent timing of framing processes will affect the RF combing of these data.
Therefore, a method for storing received data in a buffer for a period before scheduling is employed in the prior art to satisfy the requirement for the SFN that a number of base stations should transmit signals having the same content simultaneously at the same frequency. For example, if a packet is transmitted from a gateway to two base stations simultaneously with the time at which it arrives at these two base stations being 7:55 and 8:01, respectively, then each of these base stations stores the received packet and transmits it at a later time (at 8:30, for example). In this way, it can be ensured that the base stations transmit the packets with the same content at the same time.
The above illustrative example is intended to explain the method for satisfying SFN requirement in the prior art. That is, the base station stores received data in a buffer and performs scheduling, after a certain period, in a first in first out manner by extracting data at the head of a queue (the data first entering the buffer). In this way, the synchronization between data contents extracted at the same time by different base stations can be ensured.
With the development of communication technology and the research of 3G standards, however, there is a new requirement for service multiplexing. That is, a number of EMBMS services dynamically share the same time-frequency resource block and the portions of the same time-frequency resource block occupied by individual services are allocated by individual base stations independently. Meanwhile, in order to satisfy the requirement for SFN, the resource allocations for the individual services have to be consistent among the respective base stations. To satisfy the above requirement for multiplexing, 3GPP introduces a concept of scheduling period. Each base station has a synchronized scheduling period within which resources are shared with other base stations. The base station schedules once for one scheduling period. When scheduling, the base station allocates corresponding resources to the individual services based on buffered data, available shared resources and a particular scheduling algorithm.
With respect to the requirements for service multiplexing and synchronization, the above prior art solution has the following drawbacks.
As noted above, due to various network delays and other delays, the same data packet transmitted from the gateway arrives at respective base stations at different time. Although the prior art solution can guarantee the synchronization between the data at the respective heads of the buffered queues in corresponding base stations, the synchronization between the respective service data received by the base stations cannot be guaranteed.
For example, if there are 100 bits of data for service A buffered in a base station and 200 bits of data for service A buffered in another base station, it will be difficult to synchronize the resource allocations for the data of service A between these two base stations. In other words, the above requirements for service multiplexing and synchronization cannot be satisfied.